1. Statement of the Technical Field
The present invention relates to the field of wireless communications. More specifically the present invention relates to pilot designs for frequency division multiple access (“FDMA”) wireless communications.
2. Description of the Related Art
There are a number of technologies currently used for wireless cellular communications. One of these is single carrier FDMA (“SC-FDMA”) in which the carrier supported by a base station is divided so that uplink (mobile station to base station) and downlink (base station to mobile station) communication with mobile stations (also referred to herein as user elements (“UE”)) is based on an assigned sub-band within the larger carrier, i.e., an assigned frequency slot and bandwidth.
The basic uplink transmission scheme for SC-FDMA is based on low-peak to average power ratio (“PAPR”) transmission that uses a cyclic prefix to achieve uplink inter-user orthogonality and to enable efficient frequency-domain equalization at the receiver side In general, there are two types of frequency division multiple access multiplexing schemes for single carrier access. One is localized FDMA where each UE occupies a different sub-band, i.e., transmissions where discrete fourier transform outputs are mapped to consecutive but discrete sub-carriers. Another is interleaved FDMA (“IFDMA”) where the discrete fourier transform output from each UE uses a comb-shaped frequency spectrum.
Dedicated pilots are needed for each UE to facilitate uplink (“UL”) transmissions. Pilot channels are “overhead” channels used to facilitate synchronization and system timing between the mobile station and the base station. Pilot channels are typically arranged to repeat a simple signal at high power level (with respect to the other channels) so that the mobile devices can locate the base station. The pilot design is, however, more challenging for single carrier based UL communications than for orthogonal FDMA (“OFDMA”) based UL access where scattered distributed pilots can be embedded into the data burst.
Given the specific requirements of FDMA and IFDMA, prior art pilot insertion schemes are not as effective when used in these arrangements. For example, the use of an UL scattered distributed pilot scheme, where pilots are embedded in each UL cluster, increases the PAPR. Known time division multiplexed (“TDM”) based schemes, while having a lower PAPR than scattered schemes, result in excessive overhead when supporting high speed mobile devices. Furthermore, it is difficult to obtain optimal pilot densities, in time and frequency, for TDM based pilots because there is overly dense pilots in frequency vs. overly sparse pilots in time. An example of a prior art arrangement is shown in FIG. 1 in which the pilot 4 is positioned at the beginning of transmission time duration, also referred to herein as the transmission time interval (“TTI”), and occupies an entire payload block 6. Data payload blocks 7 are shown as unfilled payload blocks 6.
A need exists, therefore, for an improved pilot design for FDMA communications.